2.1.1752 Part 2 Section 10.6, Relationships transform algorithm

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a.   The standard specifies that a Relationship element shall be used for signing if the Id value matches any SourceId value or if the Type value matches any SourceType value.

Office will only use Relationship elements for signing if the Id value matches any SourceId value.

The relationship transform should set TargetMode="Internal" before doing a c14n transform.